1. Field of the Invention
The present invention relates to a process for preparing carbonate compounds via a multiple-step synthesis process.
2. Description of Prior Art
Conventionally, a phosgene process and a non-phosgene process are used for producing carbonate compounds. In the phosgene process, phosgene is reacted with alcohols in the absence of catalyst at a low temperature to produce carbonate compounds; the phosgene process, however, has problems such as highly toxic phosgene is used as a starting compound, and a special manufacturing facility required to cope with the highly corrosive by-product (hydrochloric acid) generated by the process; such facility is very costly. Furthermore, extremely high amount of chlorine is often contained in the resulting carbonate products.
The non-phosgene process can be roughly divided into alcohols/carbon monooxide oxidation process, alkyl nitrite process and transesterification process. These processes will be described in detail as follows:
The alcohols/carbon monooxide oxidation process can be further divided into a liquid phase process and a gas phase process; for the liquid phase process as described in the U.S. Pat. No. 4,218,391, a high reaction pressure and a corrosive catalyst are required; therefore, special facilities are in need and such facilities are very costly for the process. In addition, the resulting carbonate products always contain a large amount of water and a great deal of energy is wasted in purification process for removing the water from the carbonate products. The gas phase process as described in the U.S. Pat. No. 3,114,762 avoids the need of high reaction pressure in the liquid phase process; however, because heterogeneous catalyst is used the yield of product is low, and the catalyst used in the process is toxic. PA1 1. By using the urea derivatives as starting material, the cost of raw material is reduced; PA1 2. The use of noble metal as catalyst is optional such that the cost of catalyst is reduced when non-noble metal is employed as catalyst; PA1 3. Homogeneous catalysts are used in this invention which results in a high selectivity for the reaction; the catalysts are not corrosive, and can be recovered for re-use; PA1 4. The reaction is carried out under low pressure and liquid phase, therefore, problems existing in a high pressure reaction such as high degree of danger and high cost in facility as well as in operation are eliminated; and PA1 5. No water is produced throughout the process reactions of the present invention, so the catalyst will be free from being poisoned by the combination of water and the catalyst, and side reactions which adversely affect the cost of the purification of the resulting alkyl carbonates can be avoided.
The alkyl nitrite process also includes a liquid phase process and a gas phase process. The liquid phase process as described in the U.S. Pat. No. 4,229,589 relates to the reaction of carbon monooxide with alkyl nitrite in the presence of metallic palladium as catalyst under a high pressure to obtain carbonate compounds. Nevertheless, there are problems in this process such that the yield is, low and a costly facility is needed. The gas phase process as described in the U.S. Pat. No. 4,229,591 employs the catalytic reaction of carbon monooxide with alkyl nitrite, which reaction is carried out under a low pressure and gas phase. The disadvantage with the above process is that the cost for recycling the alkyl nitrite from the resulting products is high.
The transesterification process can be further divided into the following two processes according to the starting materials used: one is the process (see U.S. Pat. No. 4,434,105) which involves the catalytic reaction of alcohols, ethylene oxide and carbon dioxide in liquid phase; and another one is the process (see U.S. Pat. No. 4,062,884) which involves the catalytic reaction of alcohols and ethylene carbonate in liquid phase. The disadvantages of these processes are such that the cost of the preparation of ethylene oxide or ethylene carbonate, to be used as the starting material, is high and, in order to obtain a high selectivity, the use of a large amount of alcohols is required with a high reflux ratio as a result the operating cost increases.
Under the circumstances mentioned above, attempts had been made in order to overcome the disadvantages described above through the inventors' extensive and intensive studies, and the present invention was thus completed.